Optical head apparatus and optical information recording and reproduction apparatus

ABSTRACT

Disclosed is an optical head apparatus comprising: a light source; a collimating means of converting a beam of light emitted from the light source into a substantially parallel beam of light; a focusing means of focusing the light onto an information medium surface; a beam splitting means of splitting the beam of light modulated by the information medium; and a light receiving means of receiving the light modulated by the information medium, wherein a lens having a negative power and a lens having a positive power are arranged in this order as viewed from the collimating means side between the collimating means and the focusing means, and at least either one of the lenses is moved along an optical axis to correct spherical aberration occurring on the information medium surface, and wherein the distance from the lens having the positive power to the focusing means is set substantially equal to the focal length of the lens having the positive power.

This application is a continuation of U.S. patent application Ser. No.10/486,003, filed Jan. 19, 2005, which is a U.S. National PhaseApplication of PCT International Application PCT/JP2002/08051, filedAug. 7, 2002, the entire disclosure of which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to an optical head apparatus for a digitalvideo disk, a digital audio disk, an optical memory disk for a computer,and the like. The present invention relates more particularly to anoptical head apparatus having the function of correcting the sphericalaberration of an objective lens, and an optical information recordingand reproduction apparatus.

BACKGROUND ART

To increase the recording density of optical disks, generally the lightsource wavelength has to be reduced or the NA of the objective lens hasto be increased. For DVD optical disks, a wavelength of 650 nm and NA of0.6 have been used, but it would be possible to increase the recordingdensity by using a blue emitting light source with a wavelength of 400nm and increasing the NA of the objective lens up to 0.85. With such anoptical head, however, a slight error in optical disk substratethickness would result in a large spherical aberration.

For example, in the case of the first-described DVD optical system, asubstrate thickness error of 10 μm results in an RMS sphericalaberration of about 0.01 λ, but with the latter optical systemconditions, the same substrate thickness error of 10 μm would result ina spherical aberration of about 0.1 λ which is 10 times larger.

Spherical aberration correcting optical systems have been proposed forcorrecting such spherical aberration. For example, Japanese PatentApplication No. 2000-131603 proposes a method in which an afocal opticalsystem is constructed using two lenses, a convex lens and a concavelens, and the spacing between the lenses is varied to correct sphericalaberration.

In this optical system, however, if the convex lens or the concave lensis moved along the optical axis to correct spherical aberration, therearises a first problem, that is, the light utilization efficiencyvaries. This will be described in detail below.

A semiconductor laser or the like is used as the light source for anoptical head. The far field intensity pattern of a semiconductor laserresembles a Gaussian distribution. That is, the intensity is the higheston the optical axis, and decreases with the distance from the opticalaxis. Generally, if it is attempted to increase the amount of lightincident on an objective lens, the intensity of light at the peripheryor rim intensity of the effective diameter of the object lens decreases.If the intensity of light at the periphery or rim intensity of theeffective diameter decreases markedly, the diameter of the spot focusedby the object lens increases. Conversely, if it is attempted to obtain auniform intensity distribution within the effective diameter of theobject lens, the efficiency of gathering light from the semiconductorlaser drops. In this way, how much of the light emitted from thesemiconductor laser is to be gathered into the objective lens is acrucial parameter that affects the performance of the optical head.

However, when one of the lenses in the afocal optical system is moved tocorrect spherical aberration, the angle of the light incident on theobject lens changes when viewed from the semiconductor laser as thelight source, and hence there has been the first problem in that thelight gathering efficiency and the diameter of the spot focused by theobject lens change.

Further, in the case of a disk comprising two layers of differentthicknesses for increased storage capacity, the second layer is thickerthan the first layer. Recording/reproduction on the second layer isperformed using the light passed through the first layer. Recordedportions and non-recorded portions are unevenly distributed in the firstlayer, and this affects the recording/reproduction characteristics ofthe second layer. Hence, there arises a second problem, that is, it isdesirable that the effective NA for the second layer be made larger.

In the case of a thick disk, coma aberration that occurs when the disktilts increases. Further, the light absorption is larger than in thecase of a thin disk, hence a third problem.

DISCLOSURE OF THE INVENTION

The present invention has been devised to solve the first problem of theprior art, and an object of the invention is to prevent the lightgathering efficiency and the focused spot diameter from changing evenwhen spherical aberration is corrected.

To achieve the above first object, the present invention is an opticalhead apparatus comprising: a light source; a collimating means ofconverting a beam of light emitted from the light source into asubstantially parallel beam of light; a focusing means of focusing thelight onto an information medium surface; a beam splitting means ofsplitting the beam of light modulated by the information medium; and alight receiving means of receiving the light modulated by theinformation medium, wherein

a lens having a negative power and a lens having a positive power arearranged in this order as viewed from the collimating means side betweenthe collimating means and the focusing means, and

at least either one of the lenses is moved along an optical axis tocorrect spherical aberration occurring on the information mediumsurface, and wherein

the distance from the lens having the positive power to the focusingmeans is set substantially equal to the focal length of the lens havingthe positive power.

In the present invention, when correcting the spherical aberration of anentire optical system by moving the lens in the light path along thedirection of the optical axis and thereby varying the diverging angle ofthe beam incident on the objective lens. The height of the peripheralray or marginal ray emitted from the spherical aberration correctingoptical system decreases when the beam is more diverging from theneutral position, and increases when the beam is more converging,thereby ensuring that a uniform distribution of light intensity alwaysenters the objective lens.

Further, the present invention is an optical information recording andreproduction apparatus which is equipped with any one of theabove-described optical head apparatus, and which records information onor reproduces information from medium surface of an optical disksubstrate by using the optical head apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a)-1(b) are optical path diagrams showing one configurationexample of an optical head apparatus according to the present invention.

FIG. 2 is a schematic diagram illustrating the principle of the opticalhead apparatus according to the present invention.

FIG. 3 is a diagram showing spherical aberration in a first example ofthe optical head apparatus according to the present invention.

FIG. 4 is a diagram showing spherical aberration in a second example ofthe optical head apparatus according to the present invention.

FIG. 5 is a diagram showing spherical aberration in a third example ofthe optical head apparatus according to the present invention.

FIGS. 6( a)-6(b) are optical path diagrams showing one configurationexample of an optical head apparatus related to the present invention.

FIG. 7 is a diagram showing spherical aberration in the example of theoptical head apparatus related to the present invention.

FIG. 8 is a diagram showing the configuration of an informationrecording and reproduction apparatus and the optical head apparatusshown in the embodiment of the present invention.

FIG. 9 is a diagram showing the configuration of an informationrecording and reproduction apparatus and the optical head apparatusrelated to the present invention.

DESCRIPTION OF REFERENCE NUMERALS

-   1. PARALLEL LIGHT-   2. CONCAVE LENS-   3. DIVERGENT LIGHT-   4. CONVEX LENS-   5. PARALLEL LIGHT-   6. APERTURE OF OBJECTIVE LENS-   7. OPTICAL AXIS

BEST MODE FOR CARRYING OUT THE INVENTION EMBODIMENT 1

An optical head apparatus according to the present invention will bedescribed in detail below with reference to the drawings.

FIG. 1( a) is an optical path diagram showing the configuration of anoptical head apparatus according to the present invention. Parallellight 1 with a beam diameter of Φa enters a concave lens 2, and emergesas divergent light 3 which then enters a convex lens 4. The lightentering the convex lens is converted back into parallel light 5 whichreaches the aperture 6 of an objective lens. Here, when the convex lens4 is moved along the optical axis 7 in the direction away from theconcave lens 2 (FIG. 1( b)), since the light emerging from the concavelens 2 is divergent light, the height 8 of the ray emerging from theperiphery of the convex lens 4 becomes higher than the correspondingheight 9 of the parallel light. When the convex lens 4 is moved in thedirection away from the concave lens 2, the light emerges as convergentlight 10. Since the light whose height at the periphery is higher thanin the case of the parallel light emerges as the convergent light 10,the light reaches the aperture 6 of the objective lens just in the caseof the parallel light, depending on the condition. That is, even whenthe convex lens 4 is moved to correct spherical aberration, the parallellight 1 with the beam diameter of Φa enters the objective lens whilemaintaining its original intensity distribution.

Here, the condition that does not cause any change in the intensitydistribution when the lens is moved to correct spherical aberration willbe described in detail with reference to drawings. FIG. 2 is a schematicdiagram illustrating the principle of the optical head apparatusaccording to the present invention. When the concave lens 11 and theconvex lens 12 are placed in an afocal optical configuration, light 13entering as parallel light emerges also as parallel light 14. The focallength of the convex lens 12 is denoted by fp. The peripheral ray ormarginal ray height before the convex lens 12 is moved is denoted by c.When the convex lens 12 is moved along the optical axis by Δd in thedirection away from the concave lens 11, the peripheral ray or marginalray height increases by Δd•c/fp. The light emerges from the convex lens12 as convergent light 15 which is brought to a focus at a distance of bfrom the convex lens. Paraxial imaging in this condition is given by theequation1/(fp+Δd)+1/b=1/fpIf the peripheral ray or marginal ray height at the aperture 16 of theobjective lens located at a distance of s from the convex lens 12 is tobe made the same as the height c of the parallel light, the followingcondition should be satisfied.(c•Δd/fp)/s=(c+c•Δd/fp)/bFrom the above equation, the relations=fpis obtained.

Therefore, it can be seen that, when the distance from the convex lens12 to the aperture 16 of the objective lens is set equal to the focallength of the convex lens 12, the same intensity distribution and thesame light utilization efficiency can be obtained at all times.

Here, since the convex lens 12 is moved along the optical axis, thedistance from the convex lens to the aperture of the objective lenscannot be maintained at all times equal to the focal length of theconvex lens.

However, the amount of movement, Δd, of the convex lens is sufficientlysmall compared with its focal length. Accordingly, the variation of thedistance, s, from the convex lens to the aperture 16 of the objectivelens with the movement of the convex lens can be regarded as beingsufficiently small compared with the focal length of the convex lens.

Specific numerical examples according to a first embodiment of thepresent invention will be shown below. First, numerical examples for theobjective lens used in common among the various numerical examples areshown. The objective lens comprises two elements in two groups, with thesurface designated as the first surface, the second surface, the thirdsurface, and the fourth surface as viewed from the spherical aberrationcorrecting optical system. The optical disk comprises a plane parallelplate.

fb: Focal length of objective lens (mm)

Rb1: Radius of curvature of the first surface (mm)

Rb2: Radius of curvature of the second surface (mm)

Rb3: Radius of curvature of the third surface (mm)

Rb4: Radius of curvature of the fourth surface (mm)

Eb1: Spacing between the first lens and the second lens (mm)

db1: Thickness of the first lens (mm)

db2: Thickness of the second lens (mm)

nb1: Refractive index of the first lens for the operating wavelength

nb2: Refractive index of the second lens for the operating wavelength

ncd: Refractive index of the optical disk substrate for the operatingwavelength

tc: Thickness of the optical disk transparent substrate on theinformation recording surface

W: Operating wavelength (nm)

fb=2.000

Rb1=1.900

Rb2=−7.800

Rb3=0.99466

Rb4: Plane

Eb1=1.150

db1=1.2

db2=1.0535

nb1=1.52331

nb2=1.52331

ncd=1.61736

tc=0.1

WD=0.30

W=405

Aspherical shape is given by the following equation (1)

$\begin{matrix}{X = {\frac{{Cjh}^{2}}{1 + \sqrt{1 - {\left( {1 + {Kj}} \right){Cj}^{2}h^{2}}}} + {\sum\limits_{\;}^{\;}{A_{j,n}h^{n}}}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

The parameters in the above equation have the following meanings.

X: Distance from the tangential plane on the vertex of the asphericalsurface to a point lying on the aspherical surface and having a heightof h from the optical axis

h: Height from the optical axis

Cj: Curvature of the j-th surface at the vertex of the asphericalsurface (Ci=1/Rj)

Kj: Conic constant of the j-th surface

Aj,n: n-th order aspherical coefficient of the j-th surface

Of the coefficients, only the aspherical coefficients of the objectivelens are suffixed with “b”.

kb1=−4.593396e−001

Ab1,4=8.386037e−004

Ab1,6=−2.243728e−004

Ab1,8=−1.594195e−004

Ab1,10=2.620701e−005

Ab1,12=−2.738825e−005

kb2=1.272600e+001

Ab2,4=−3.117861e−003

Ab2,6=−1.073863e−003

Ab2,8=1.258618e−004

Ab2,10=−1.951853e−004

Ab2,12=4.18124e−005

kb3=−6.080135e−001

Ab3,4=3.894679e−002

Ab3,6=1.463247e−002

Ab3,8=1.262713e−002

Ab3,10=−1.226316e−002

Ab3,12=1.779569e−002

Next, numerical data are shown for the lenses constructed to correct thespherical aberration of the optical head optics. In the numericalexamples given below, the lenses are designated as the first lens andthe second lens as viewed from the light source side, and in each lens,the surface nearer to the light source is designated as the firstsurface, and the surface nearer to the objective lens as the secondsurface.

fp: Focal length of the convex lens (mm)

R11: Radius of curvature of the first surface of the first lens (mm)

R12: Radius of curvature of the second surface of the first lens (mm)

R21: Radius of curvature of the first surface of the second lens (mm)

R22: Radius of curvature of the second surface of the second lens (mm)

E1: Spacing between the first lens and the second lens (mm)

d1: Thickness of the first lens (mm)

d2: Thickness of the second lens (mm)

n1: Refractive index of the first lens for the operating wavelength

n2: Refractive index of the second lens for the operating wavelength

EFF: Diameter of the emergent beam on the objective lens side

EXAMPLE 1

fp=10.0

R11=−8.48

R12=14.3

R21=33.2

R22=−9.02

E1=2.0

d1=1.2

d2=1.2

n1=1.74188

n2=1.71791

EFF=3.0

Here, since the focal length of the convex lens is 10 mm, the distancefrom the convex lens to the first surface of the objective lens is alsoset to 10 mm, assuming that the aperture of the objective lens lies inthe first surface of the objective lens. See FIG. 3.

The amount of third-order spherical aberration corrected when the convexlens is moved by ±0.5 mm is about 0.18 λ: RMS. In the followingdescription, when the convex lens is moved toward the light source, theamount of movement is designated as negative, and when the convex lensis moved toward the objective lens, the amount of movement is designatedas positive.

The height of the light ray incident on the spherical aberrationcorrecting optical system, which corresponds to the peripheral ray ormarginal ray incident on the objective lens, is shown below along withthe amount of movement of the convex lens when the distance from theconvex lens to the objective lens is set to 10 mm.

Amount of movement of convex lens Incident ray height −0.5 mm 1.0426 mm  0.0 mm 1.0522 mm +0.5 mm 1.0568 mm

Thus, it can be seen that when the distance from the convex lens to theobjective lens is set substantially equal to the focal length of theconvex lens, the variation of the incident ray height with the movementof the convex lens can be reduced to a very small value. The amount ofvariation of the incident ray height is less than 1%, which means thatthe incident ray height remains substantially unchanged; therefore, itcan be said that substantially no change occurs in the light utilizationefficiency as well as in the diameter of the spot focused by theobjective lens.

EXAMPLE 2

fp=20.0

R11=−23.11

R12=24.93

R21=81.09

R22=−12.036

E1=2.5

d1=1.2

d2=2.0

n1=1.74188

n2=1.52801

EFF=3.0

Here, since the focal length of the convex lens is 20 mm, the distancefrom the convex lens to the first surface of the objective lens is alsoset to 20 mm, assuming that the aperture of the objective lens lies inthe first surface of the objective lens. See FIG. 4.

The amount of third-order spherical aberration corrected when the convexlens is moved by ±1.5 mm is about 0.15 λ: RMS. In the followingdescription, when the convex lens is moved toward the light source, theamount of movement is designated as negative, and when the convex lensis moved toward the objective lens, the amount of movement is designatedas positive.

The height of the light ray incident on the spherical aberrationcorrecting optical system, which corresponds to the peripheral ray ormarginal ray incident on the objective lens, is shown below along withthe amount of movement of the convex lens when the distance from theconvex lens to the objective lens is set to 20 mm.

Amount of movement of convex lens Incident ray height −1.5 mm 1.1897 mm  0.0 mm 1.1965 mm +1.5 mm 1.2016 mm

Thus, it can be seen that when the distance from the convex lens to theobjective lens is set substantially equal to the focal length of theconvex lens, the variation of the incident ray height with the movementof the convex lens can be reduced to a very small value. The amount ofvariation of the incident ray height is less than 0.6%, which means thatthe incident ray height remains substantially unchanged; therefore, itcan be said that substantially no change occurs in the light utilizationefficiency as well as in the diameter of the spot focused by theobjective lens.

EXAMPLE 3

fp=8.0

R11=−6.914

R12=5.35

R21=71.645

R22=−4.414

E1=3.0

d1=0.8

d2=1.5

n1=1.75747

n2=1.52331

EFF=3.7

The second surface of the second lens is aspherical, and its asphericalshape is also expressed by (equation 1). See FIG. 5.

k2=−0.1710231

A2,4=2.839637×10⁻⁵

Here, since the focal length of the convex lens is 8 mm, the distancefrom the convex lens to the first surface of the objective lens is alsoset to 8 mm, assuming that the aperture of the objective lens lies inthe first surface of the objective lens.

The amount of third-order spherical aberration corrected when the convexlens is moved by ±0.25 mm is about 0.25 λ: RMS. In the followingdescription, when the convex lens is moved toward the light source, theamount of movement is designated as negative, and when the convex lensis moved toward the objective lens, the amount of movement is designatedas positive.

The height of the light ray incident on the spherical aberrationcorrecting optical system, which corresponds to the peripheral ray ormarginal ray incident on the objective lens, is shown below along withthe amount of movement of the convex lens when the distance from theconvex lens to the objective lens is set to 8 mm.

Amount of movement of convex lens Incident ray height −0.25 mm 0.8182 mm   0.0 mm 0.8208 mm +0.25 mm 0.8372 mm

Thus, it can be seen that when the distance from the convex lens to theobjective lens is set substantially equal to the focal length of theconvex lens, the variation of the incident ray height with the movementof the convex lens can be reduced to a very small value. The amount ofvariation of the incident ray height is less than 0.3%, which means thatthe incident ray height remains substantially unchanged; therefore, itcan be said that substantially no change occurs in the light utilizationefficiency as well as in the diameter of the spot focused by theobjective lens.

Next, one example of an optical head apparatus related to the presentinvention and invented by the present inventor will be described indetail with reference to drawing.

In the foregoing first embodiment, an afocal optical system comprising aconcave lens and a convex lens has been used as the spherical aberrationcorrecting optical system, but in the second embodiment describedhereinafter, a single collimating lens which converts semiconductorlaser light into parallel light is used as the spherical aberrationcorrecting optical system.

FIG. 6 is an optical path diagram illustrating the principle of thepresent embodiment. Light 18 emitted from a semiconductor laser 17 isconverted by the collimating lens 19 into parallel light 20 which entersthe aperture 21 of the objective lens. When the collimating lens 19 ismoved along the optical axis 22 in the direction away from thesemiconductor laser 17 (FIG. 6( b)), since the light emitted from thesemiconductor laser 17 is divergent light, the height 23 of the rayemerging from the periphery of the collimating lens 19 becomes higherthan the corresponding height 24 of the parallel light. When thecollimating lens 19 is moved in the direction away from thesemiconductor laser 17, the light emerges as convergent light 25.

Since the light whose height at the periphery is higher than in the caseof the parallel light emerges as the convergent light 25, the lightreaches the aperture 21 of the objective lens just as in the case of theparallel light, depending on the condition. That is, even when thecollimating lens 19 is moved to correct spherical aberration, the lightemitted from the semiconductor laser 17 enters the objective lens whilemaintaining its original intensity distribution.

If the convex lens shown in FIG. 2 is replaced by the collimating lens,the condition that no change is caused in the intensity distributionwhen the lens is moved to correct spherical aberration can be explainedin exactly the same way. Therefore, it can be seen that, when thedistance, s, from the collimating lens to the aperture of the objectivelens is set equal to the focal length fc of the collimating lens, thatis,s=fcthen the same intensity distribution and the same light utilizationefficiency can be obtained at all times from the semiconductor laser.

Here, since the collimating lens is moved along the optical axis, thedistance from the collimating lens to the aperture of the objective lenscannot be maintained at all times equal to the focal length of thecollimating lens.

However, the amount of movement, Δd, of the collimating lens issufficiently small compared with its focal length. Accordingly, thevariation of the distance, s, from the collimating lens to the apertureof the objective lens with the movement of the collimating lens can beregarded as being sufficiently small compared with the focal length ofthe collimating lens.

Specific numerical examples according to the present embodiment will bedescribed below. The parameters excluding those for the collimatinglens, but including those for the objective lens, are the same as thosein the first embodiment.

EXAMPLE 4

Specific numerical data in the fourth example are shown below. Thesurfaces of the collimating lens are designated as the first surface andthe second surface as viewed from the light source side.

fc: Focal length of the collimating lens (mm)

R1: Radius of curvature of the first surface of the collimating lens(mm)

R2: Radius of curvature of the second surface of the collimating lens(mm)

d1: Thickness of the collimating lens (mm)

n1: Refractive index of the collimating lens for the operatingwavelength

EFF: Diameter of the emergent beam on the objective lens side

fc=16.0

R1: Plane

R2=−10.72704

d1=2

n1=1.67044

EFF=3.4

Here, since the focal length of the collimating lens is 16 mm, thedistance from the collimating lens to the first surface of the objectivelens is also set to 16 mm, assuming that the aperture of the objectivelens lies in the first surface of the objective lens. See FIG. 7.

The amount of third-order spherical aberration corrected when thecollimating lens is moved by ±0.8 mm is about 0.15 λ: RMS. In thefollowing description, when the collimating lens is moved toward thelight source, the amount of movement is designated as negative, and whenthe collimating lens is moved toward the objective lens, the amount ofmovement is designated as positive.

The angle of the light ray incident on the collimating lens, whichcorresponds to the peripheral ray or marginal ray incident on theobjective lens, is shown below along with the amount of movement of thecollimating lens when the distance from the collimating lens to theobjective lens is set to 16 mm.

Amount of movement of collimating lens Incident ray angle −0.8 mm6.06097 degrees   0.0 mm 6.09702 degrees +0.8 mm 6.10389 degrees

Thus, it can be seen that when the distance from the collimating lens tothe objective lens is set substantially equal to the focal length of thecollimating lens, the variation of the incident ray angle with themovement of the collimating lens can be reduced to a very small value.The amount of variation of the incident ray angle is less than 0.6%,which means that the incident ray angle remains substantially unchanged;therefore, it can be said that substantially no change occurs in thelight utilization efficiency as well as in the diameter of the spotfocused by the objective lens.

FIG. 8 is a diagram showing the configuration of an optical informationrecording and reproduction apparatus and an optical head apparatusaccording to the present invention that uses the optical system shown inthe foregoing first embodiment. In FIG. 8, a beam of light emitted froma semiconductor laser 26, the light source, passes through a beamsplitter 27 and is made substantially parallel by a collimating lens 28.The light further passes through a concave lens 29 and a convex lens 30,and emerges again as a substantially parallel beam of light which isthen focused onto an information medium surface 32 a of an optical disk32 by the action of an objective lens 31 that comprises two elements intwo groups as shown in the first to fourth examples. The focused spot isdiffracted by pits and lands formed on the information medium surface 32a. The laser light diffracted and reflected by the information mediumsurface 32 a passes through the objective lens 31, the convex lens 30,the concave lens 29, and the collimating lens 29, and is reflected bythe beam splitter 27 into a detection lens 33 where the light isrefracted and focused onto a photodetector 34. Based on an electricalsignal output from the photodetector 34, a change in the intensity oflight modulated by the information medium surface 32 a is detected,thereby reading the data.

Here, the convex lens 30 is mounted in such a manner as to be movablealong the optical axis and thereby correct the spherical aberrationassociated with the substrate thickness of the optical disk 32 and otheroptical head optics. When the distance from the convex lens to theobjective lens 31 is set substantially equal to the focal length of theconvex lens 30, the beam of light emitted from the semiconductor laser26 enters the objective lens 31 while maintaining a constant beam sizeeven if the convex lens 30 is moved along the optical axis.

In this way, not only the semiconductor laser light utilization in theobjective lens but also the intensity of light incident on the peripheryof the objective lens is maintained constant; as a result, the shape ofthe spot focused on the medium surface can also be maintained constant.

FIG. 9 is a diagram showing the configuration of an optical informationrecording and reproduction apparatus and an optical head apparatusrelated to the present invention that uses the optical system shown inthe one example described above. In FIG. 9, the beam of light emittedfrom the semiconductor laser light source 26, and passed through thebeam splitter 27, is made substantially parallel by a collimating lens35. Then, the light is focused onto the information medium surface 32 aof the optical disk 32 by the action of the objective lens 31 thatcomprises two elements in two groups as shown in the first to fourthexamples. The focused spot is diffracted by pits and lands formed on theinformation medium surface 32 a. The laser light diffracted andreflected by the information medium surface 32 a passes through theobjective lens 31 and the collimating lens 35, and is reflected by thebeam splitter 27 into the detection lens 33 where the light is refractedand focused onto the photodetector 34. Based on an electrical signaloutput from the photodetector 34, a change in the intensity of lightmodulated by the information medium surface 32 a is detected, therebyreading the data.

Here, the collimating lens 35 is mounted in such a manner as to bemovable along the optical axis and thereby correct the sphericalaberration associated with the substrate thickness of the optical disk32 and other optical head optics. When the distance from the collimatinglens to the objective lens 31 is set substantially equal to the focallength of the collimating lens 35, the beam of light emitted from thesemiconductor laser 26 enters the objective lens 31 while maintaining aconstant beam size even if the collimating lens 35 is moved along theoptical axis. In this way, not only the semiconductor laser lightutilization in the objective lens but also the intensity of lightincident on the periphery of the objective lens is maintained constant;as a result, the shape of the spot focused on the medium surface canalso be maintained constant.

In the above described embodiment, the convex lens is moved along theoptical axis, but the same effect can be achieved if the concave lens ismoved rather than the convex lens.

In each example, the convex lens and the concave lens have each beendescribed as being constructed from a single lens, but alternatively,they may be constructed using lens groups of a plurality of lenses, onegroup having a positive power and the other having a negative power.Further, when constructing each lens group using a plurality of lenses,the lens group may be designed to exhibit an achromatic effect bycombining convex and concave lenses having different dispersive powers,as is well known in the art. The achromatic effect can also be providedby integrating a diffractive element into each lens. Here, chromaticaberration can be corrected by the lens groups themselves, butalternatively, the spherical aberration correcting optical system may bedesigned to correct the chromatic aberration of the entire optical headsystem.

Further, in the above embodiment, since the convex lens and the concavelens are used in combination, if each lens is constructed from a singlelens, the chromatic aberration correcting effect can be achieved byforming the convex lens from a material having small chromaticdispersion and the concave lens from a material having large chromaticdispersion.

In the above example, the collimating lens has been described as beingconstructed from a single aspherical lens, but the collimator may beconstructed from a combination of a plurality of lens elements. Thecollimating lens may be constructed from a plurality of lens elements toprovide an achromatic effect in the same manner as described above, orthe lens may be combined with a diffractive element so as to correctchromatic aberration. In that case, the chromatic aberration of thecollimating lens itself may be corrected, or alternatively, thecollimating lens may be designed to correct the chromatic aberration ofthe entire optical head system.

In the above embodiment, the optical system has been described as beingconstructed as an afocal optical system in which parallel incident lightemerges as parallel light, but the optical system may be constructed asan optical system in which either the incident light or the emergentlight or both of them are non-parallel light. For example, when theemergent light is divergent light, the objective lens should beconstructed to correct aberration for the divergent light; the same alsoapplies to the case of convergent light.

Further, in the above example, the collimating lens is designed to makethe light substantially parallel, but it may be designed so that thelight emerges from it as divergent light or convergent light. Likewise,the objective lens may be constructed from a lens aberration-correctedfor the divergent light or the convergent light.

Next, another example related to the present invention and invented bythe present inventor will be described. This example provides an opticalhead apparatus comprising: a light source; a collimating means ofconverting a beam of light emitted from the light source into asubstantially parallel beam of light; a focusing means of focusing thelight onto an information medium surface; a beam splitting means ofsplitting the beam of light modulated by the information medium; and alight receiving means of receiving the light modulated by theinformation medium, wherein a lens having a negative power and a lenshaving a positive power are arranged in this order as viewed from thecollimating means side between the collimating means and the focusingmeans, and at least either one of the lenses is moved along an opticalaxis to correct spherical aberration occurring on the information mediumsurface, and wherein the apparatus satisfies the conditionf<d<1.25fwhere f is the focal length of the lens having the positive power, and dis the distance from the lens having the positive power to the focusingmeans.

That is, when the distance from the lens having the positive power tothe focusing means is larger than the focal length of the lens, thefollowing merit is obtained. In the case of a disk comprising two layersof different thicknesses for increased storage capacity, the secondlayer is thicker than the first layer. Recording/reproduction on thesecond layer is performed using the light passed through the firstlayer. Recorded portions and non-recorded portions are unevenlydistributed in the first layer, and this affects therecording/reproduction characteristics of the second layer. It istherefore desirable that the effective NA for the second layer be madelarger. Here, when the distance from the lens having the positive powerto the focusing means is larger than the focal length of the lens asdescribed above, the effective NA for the second layer is larger, sothat the beam can be focused into a smaller spot, and the resistance tonoise thus increases.

More specifically, when using the convex lens shown in the firstexample, the distance from the convex lens to the objective lens is setin the range of about 10 mm to 12.5 mm.

The height of the light ray incident on the spherical aberrationcorrecting optical system, which corresponds to the peripheral ray ormarginal ray incident on the objective lens, is shown below along withthe amount of movement of the convex lens when the distance from theconvex lens to the objective lens is set to 12.5 mm.

Amount of movement of convex lens Incident ray height −0.5 mm 1.0293 mm  0.0 mm 1.0522 mm +0.5 mm 1.0710 mm

On the other hand, when using the convex lens shown in the secondexample, the distance from the convex lens to the objective lens is setin the range of 20 mm to 25 mm.

The height of the light ray incident on the spherical aberrationcorrecting optical system, which corresponds to the peripheral ray ormarginal ray incident on the objective lens, is shown below along withthe amount of movement of the convex lens when the distance from theconvex lens to the objective lens is set to 25 mm.

Amount of movement of convex lens Incident ray height −1.5 mm 1.1676 mm  0.0 mm 1.1965 mm +1.5 mm 1.2254 mm

Further, when using the convex lens shown in the third example, thedistance from the convex lens to the objective lens is set in the rangeof about 8 mm to 10 mm.

The height of the light ray incident on the spherical aberrationcorrecting optical system, which corresponds to the peripheral ray ormarginal ray incident on the objective lens, is shown below along withthe amount of movement of the convex lens when the distance from theconvex lens to the objective lens is set to 10 mm.

Amount of movement of convex lens Incident ray height −0.25 mm 0.8109 mm   0.0 mm 0.8208 mm +0.25 mm 0.8456 mm

As shown above, when the convex lens is moved in the negative direction,that is, in the direction opposite to the objective lens, the incidentray height decreases. That the incident ray height decreases means that,of the light rays emitted from the semiconductor laser, only the raysnearer to the center are used, and therefore that the light intensitydistribution becomes more uniform. As a result, the effective NAincreases, and the beam can be focused into a smaller spot. On the otherhand, moving the convex lens in the negative direction means correctingaberration for a thicker disk. Accordingly, when performingrecording/reproduction on the second layer of the two-layer disk, sincethe beam is focused into a smaller spot, the resistance to noise can beincreased.

In still another example related to the present invention and inventedby the present inventor, there is provided an optical head apparatuscomprising: a light source; a collimating lens which converts a beam oflight emitted from the light source into a substantially parallel beamof light; a focusing means of focusing the light onto an informationmedium surface; a beam splitting means of splitting the beam of lightmodulated by the information medium; and a light receiving means ofreceiving the light modulated by the information medium, wherein thecollimating lens is moved along an optical axis to correct sphericalaberration occurring on the information medium surface, and wherein theapparatus satisfies the conditionfc<dc<1.25fcwhere fc is the focal length of the collimating lens, and dc is thedistance from the collimating lens to the focusing means.

That is, as in the foregoing example, when the distance from thecollimating lens to the focusing means is larger than the focal lengthof the collimating lens, the effective NA for the second layer of thetwo-layer disk is larger, so that the beam can be focused into a smallerspot, and the resistance to noise thus increases.

More specifically, when using the collimating lens shown in the fourthexample, the distance from the collimating lens to the objective lens isset in the range of about 16 mm to 20 mm.

The angle of the light ray incident on the collimating lens, whichcorresponds to the peripheral ray or marginal ray incident on theobjective lens, is shown below along with the amount of movement of thecollimating lens when the distance from the collimating lens to theobjective lens is set to 20 mm.

Amount of movement of collimating lens Incident ray angle −0.8 mm5.98621 degrees   0.0 mm 6.09702 degrees +0.8 mm 6.18210 degrees

As shown above, when the collimating lens is moved in the negativedirection, that is, in the direction opposite to the objective lens, theincident ray angle decreases. That the incident ray angle decreasesmeans that, of the light rays emitted from the semiconductor laser, onlythe rays nearer to the center are used, and therefore that the lightintensity distribution becomes more uniform. As a result, the effectiveNA increases, and the beam can be focused into a smaller spot. On theother hand, moving the collimating lens in the negative direction meanscorrecting aberration for a thicker disk. Accordingly, when performingrecording/reproduction on the second layer of the two-layer disk, sincethe beam is focused into a smaller spot, the resistance to noise can beincreased.

Next, a further example related to the present invention and invented bythe present inventor will be described. This example provides an opticalhead apparatus comprising: a light source; a collimating means ofconverting a beam of light emitted from the light source into asubstantially parallel beam of light; a focusing means of focusing thelight onto an information medium surface; a beam splitting means ofsplitting the beam of light modulated by the information medium; and alight receiving means of receiving the light modulated by theinformation medium, wherein a lens having a negative power and a lenshaving a positive power are arranged in this order as viewed from thecollimating means side between the collimating means and the focusingmeans, and at least either one of the lenses is moved along an opticalaxis to correct spherical aberration occurring on the information mediumsurface, and wherein the apparatus satisfies the condition0.5f<d<fwhere f is the focal length of the lens having the positive power, and dis the distance from the lens having the positive power to the focusingmeans.

That is, when the distance from the lens having the positive power tothe focusing means is smaller than the focal length of the lens, thefollowing merit is obtained.

When the distance from the lens to the focusing means is set smallerthan the focal length of the lens, the amount of light increases in thecase of a thick disk. On the other hand, the semiconductor laser lightsource has a light intensity distribution such that the intensity is thehighest in the center and decreases toward the periphery. That is,increasing the amount of light means that the light at the periphery ofthe semiconductor laser is also gathered; as a result, the intensity ofthe peripheral ray or marginal ray incident on the objective lensdecreases. This in effect means that the NA decreases. Since comaaberration that occurs when the disk tilts increases in the case of athick disk, the NA thus decreased offers the effect of reducing theamount of coma aberration.

Further, in the case of a thick disk, light absorption is larger than inthe case of a thin disk; accordingly, when the distance from the lens tothe focusing means is set smaller than the focal length of the lens, adesirable result is obtained because the amount of light increases inthe case of a thick disk.

More specifically, when using the convex lens shown in the firstexample, the distance from the convex lens to the objective lens is setin the range of 5 mm to 10 mm.

The height of the light ray incident on the spherical aberrationcorrecting optical system, which corresponds to the peripheral ray ormarginal ray incident on the objective lens, is shown below along withthe amount of movement of the convex lens when the distance from theconvex lens to the objective lens is set to 5 mm.

Amount of movement of convex lens Incident ray height −0.5 mm 1.0705 mm  0.0 mm 1.0522 mm +0.5 mm 1.0295 mm

On the other hand, when using the convex lens shown in the secondexample, the distance from the convex lens to the objective lens is setin the range of about 10 mm to 20 mm.

The height of the light ray incident on the spherical aberrationcorrecting optical system, which corresponds to the peripheral ray ormarginal ray incident on the objective lens, is shown below along withthe amount of movement of the convex lens when the distance from theconvex lens to the objective lens is set to 10 mm.

Amount of movement of convex lens Incident ray height −1.5 mm 1.2367 mm  0.0 mm 1.1965 mm +1.5 mm 1.1569 mm

Further, when using the convex lens shown in the third example, thedistance from the convex lens to the objective lens is set in the rangeof about 4 mm to 8 mm.

The height of the light ray incident on the spherical aberrationcorrecting optical system, which corresponds to the peripheral ray ormarginal ray incident on the objective lens, is shown below along withthe amount of movement of the convex lens when the distance from theConvex lens to the objective lens is set to 4 mm.

Amount of movement of convex lens Incident ray height −0.25 mm 0.8333 mm0.0 mm 0.8208 mm +0.25 mm 0.8212 mm

As shown above, when the convex lens is moved in the negative direction,that is, in the direction opposite to the objective lens, the incidentray height increases. That the incident ray height increases means thatthe light emitted from the semiconductor laser is used in a wider range,and therefore that the intensity of light at the periphery drops.Accordingly, the effective NA decreases. On the other hand, moving theconvex lens in the negative direction means correcting aberration for athicker disk. As the disk thickness increases, the coma aberration thatoccurs when the disk tilts increases, affecting therecording/reproduction characteristics. At this time, since theeffective NA decreases, the coma aberration caused by disk tilt can bereduced.

In a still further example related to the present invention and inventedby the present inventor, there is provided an optical head apparatuscomprising: a light source; a collimating lens which converts a beam oflight emitted from the light source into a substantially parallel beamof light; a focusing means of focusing the light onto an informationmedium surface; a beam splitting means of splitting the beam of lightmodulated by the information medium; and a light receiving means ofreceiving the light modulated by the information medium, wherein thecollimating lens is moved along an optical axis to correct sphericalaberration occurring on the information medium surface, and wherein theapparatus satisfies the condition0.5fc<dc<fcwhere fc is the focal length of the collimating lens, and dc is thedistance from the collimating lens to the focusing means.

That is, when the distance from the collimating lens to the focusingmeans is smaller than the focal length of the lens, a similar merit tothat described above is obtained. That is, a desirable effect can beobtained in relation to light absorption.

More specifically, when using the collimating lens shown in the fourthexample, the distance from the collimating lens to the objective lens isset in the range of about 8 mm to 16 mm.

The angle of the light ray incident on the collimating lens, whichcorresponds to the peripheral ray or marginal ray incident on theobjective lens, is shown below along with the amount of movement of thecollimating lens when the distance from the collimating lens to theobjective lens is set to 8 mm.

Amount of movement of collimating lens Incident ray angle −0.8 mm6.21621 degrees 0.0 mm 6.09702 degrees +0.8 mm 5.95344 degrees

As shown above, when the collimating lens is moved in the negativedirection, that is, in the direction opposite to the objective lens, theincident ray angle increases. That the incident ray angle increasesmeans that the light emitted from the semiconductor laser is used in awider range, and therefore that the intensity of light at the peripherydrops. Accordingly, the effective NA decreases. On the other hand,moving the convex lens in the negative direction means correctingaberration for a thicker disk. As the disk thickness increases, the comaaberration that occurs when the disk tilts increases, affecting therecording/reproduction characteristics. At this time, since theeffective NA decreases, the coma aberration caused by disk tilt can bereduced.

Essential patentable portions of the above described examples related tothe present invention and invented by the present inventor will bedisclosed below.

The first aspect of the invention provides an optical head apparatuscomprising: a light source; a collimating lens which converts a beam oflight emitted from the light source into a substantially parallel beamof light; a focusing means of focusing the light onto an informationmedium surface; a beam splitting means of splitting the beam of lightmodulated by the information medium; and a light receiving means ofreceiving the light modulated by the information medium, wherein thecollimating lens is moved along an optical axis to correct sphericalaberration occurring on the information medium surface, and wherein thedistance from the collimating lens to the focusing means is setsubstantially equal to the focal length of the collimating lens.

In this way, in the first aspect of the invention, when correcting thespherical aberration of the entire optical system by moving the lens inthe light path along the direction of the optical axis and therebyvarying the diverging angle of the beam incident on the objective lens,the height of the peripheral ray or marginal ray emitted from thespherical aberration correcting optical system decreases when the beamis more diverging from the neutral position, and increases when the beamis more converging, thereby ensuring that a uniform distribution oflight intensity always enters the objective lens.

The second aspect of the invention provides an optical head apparatuscomprising: a light source; a collimating means of converting a beam oflight emitted from the light source into a substantially parallel beamof light; a focusing means of focusing the light onto an informationmedium surface; a beam splitting means of splitting the beam of lightmodulated by the information medium; and a light receiving means ofreceiving the light modulated by the information medium, wherein a lenshaving a negative power and a lens having a positive power are arrangedin this order as viewed from the collimating means side between thecollimating means and the focusing means, and at least either one of thelenses is moved along an optical axis to correct spherical aberrationoccurring on the information medium surface, and wherein the apparatussatisfies the conditionf<d<1.25fwhere f is the focal length of the lens having the positive power, and dis the distance from the lens having the positive power to the focusingmeans.

The second aspect of the invention has been devised to solve the secondproblem of the prior art, and can overcome any possible adverse effectsthat may be caused to the recording and reproduction characteristics ofthe second layer. Furthermore, with the above numerical range, theeffect that a uniform distribution of light intensity always enters theobjective lens can be achieved, if not fully, but to the extent that noproblems is caused in practice.

The third aspect of the invention provides an optical head apparatuscomprising: a light source; a collimating lens which converts a beam oflight emitted from the light source into a substantially parallel beamof light; a focusing means of focusing the light onto an informationmedium surface; a beam splitting means of splitting the beam of lightmodulated by the information medium; and a light receiving means ofreceiving the light modulated by the information medium, wherein thecollimating lens is moved along an optical axis to correct sphericalaberration occurring on the information medium surface, and wherein theapparatus satisfies the conditionfc<dc<1.25fcwhere fc is the focal length of the collimating lens, and dc is thedistance from the collimating lens to the focusing means.

The third aspect of the invention has been devised to solve the secondproblem of the prior art, and can overcome any possible adverse effectsthat may be caused to the recording and reproduction characteristics ofthe second layer. Furthermore, with the above numerical range, theeffect that a uniform distribution of light intensity always enters theobjective lens can be achieved, if not fully, but to the extent noproblem is caused in practice.

The fourth aspect of the invention provides an optical head apparatuscomprising: a light source; a collimating means of converting a beam oflight emitted from the light source into a substantially parallel beamof light; a focusing means of focusing the light onto an informationmedium surface; a beam splitting means of splitting the beam of lightmodulated by the information medium; and a light receiving means ofreceiving the light modulated by the information medium, wherein a lenshaving a negative power and a lens having a positive power are arrangedin this order as viewed from the collimating means side between thecollimating means and the focusing means, and at least either one of thelenses is moved along an optical axis to correct spherical aberrationoccurring on the information medium surface, and wherein the apparatussatisfies the condition0.5f<d<fwhere f is the focal length of the lens having the positive power, and dis the distance from the lens having the positive power to the focusingmeans.

The fourth aspect of the invention has been devised to solve the thirdproblem of the prior art, and can alleviate the adverse effectsassociated with the coma aberration and light absorption; furthermore,with the above numerical range, the effect that a uniform distributionof light intensity always enters the objective lens can be achieved, ifnot fully, but to the extent that no problem is caused in practice.

The fifth aspect of the invention provides an optical head apparatuscomprising: a light source; a collimating lens which converts a beam oflight emitted from the light source into a substantially parallel beamof light; a focusing means of focusing the light onto an informationmedium surface; a beam splitting means of splitting the beam of lightmodulated by the information medium; and a light receiving means ofreceiving the light modulated by the information medium, wherein thecollimating lens is moved along an optical axis to correct sphericalaberration occurring on the information medium surface, and wherein theapparatus satisfies the condition0.5fc<dc<fcwhere fc is the focal length of the collimating lens, and dc is thedistance from the collimating lens to the focusing means.

The fifth aspect of the invention has been devised to solve the thirdproblem of the prior art, and can alleviate the adverse effectsassociated with the coma aberration and light absorption; furthermore,with the above numerical range, the effect that a uniform distributionof light intensity always enters the objective lens can be achieved, ifnot fully, but to the extent that no problem is caused in practice.

As described above, according to the present invention, the lightgathering efficiency and the focused spot diameter can be prevented fromchanging even when spherical aberration is corrected.

The invention can also provide an optical information recording andreproduction apparatus having an effect equivalent to the above.

1. An optical system comprising: a collimating lens that converts a beamof light emitted from a light source into a substantially parallel beamof light; and an objective lens that focuses the beam of light from thecollimating lens onto an information medium surface, wherein a distancefrom the collimating lens to the objective lens is equal to a focallength of the collimating lens.
 2. An optical system comprising: acollimating lens that converts a beam of light emitted from a lightsource into a substantially parallel beam of light; an objective lensthat focuses the beam of light from the collimating lens onto aninformation medium surface; and an aperture provided at a light sourceside of the objective lens, wherein a distance from the collimating lensto the aperture is equal to a focal length of the collimating lens. 3.An optical system comprising: a collimating lens that converts a beam oflight emitted from a light source into a substantially parallel beam oflight; and an objective lens that focuses the beam of light from thecollimating lens onto an information medium surface, wherein the opticalsystem satisfies the condition:fc<dc<1.25fc where fc is the focal length of the collimating lens, anddc is a distance from the collimating lens to the objective lens.
 4. Anoptical system comprising: a collimating lens that converts a beam oflight emitted from a light source into a substantially parallel beam oflight; and an objective lens that focuses the beam of light from thecollimating lens onto an information medium surface, wherein the opticalsystem satisfies the condition:0.5 fc<dc<fc where fc is the focal length of the collimating lens, anddc is a distance from the collimating lens to the objective lens.
 5. Theoptical system as claimed in any one of claims 1 through 4, wherein thecollimating lens is movable along an optical axis.
 6. An optical headapparatus comprising: a light source emitting a beam of light; anoptical system that focuses the beam of light emitted from the lightsource onto an information medium surface, the optical system including,a collimating lens that converts a beam of light emitted from a lightsource into a substantially parallel beam of light; and an objectivelens that focuses the beam of light from the collimating lens onto aninformation medium surface, wherein a distance from the collimating lensto the objective lens is equal to a focal length of the collimatinglens.
 7. An optical head apparatus comprising: a light source emitting abeam of light; an optical system that focuses the beam of light emittedfrom the light source onto an information medium surface, the opticalsystem including, a collimating lens that converts a beam of lightemitted from a light source into a substantially parallel beam of light;an objective lens that focuses the beam of light from the collimatinglens onto an information medium surface; an aperture provided at a lightsource side of the objective lens, wherein a distance from thecollimating lens to the aperture is equal to a focal length of thecollimating lens.
 8. An optical head apparatus comprising: a lightsource emitting a beam of light; an optical system that focuses the beamof light emitted from the light source onto an information mediumsurface, the optical system including, a collimating lens that convertsa beam of light emitted from a light source into a substantiallyparallel beam of light; and an objective lens that focuses the beam oflight from the collimating lens onto an information medium surface,wherein the optical system satisfies the condition:fc<dc<1.25fc where fc is the focal length of the collimating lens, anddc is a distance from the collimating lens to the objective lens.
 9. Anoptical head apparatus comprising: a light source emitting a beam oflight; an optical system that focuses the beam of light emitted from thelight source onto an information medium surface, the optical systemincluding, a collimating lens that converts a beam of light emitted froma light source into a substantially parallel beam of light; and anobjective lens that focuses the beam of light from the collimating lensonto an information medium surface, wherein the optical system satisfiesthe condition:0.5fc<dc<fc where fc is the focal length of the collimating lens, and dcis a distance from the collimating lens to the objective lens.
 10. Theoptical head apparatus as claimed in any one of claims 6 through 9,wherein the collimating lens is movable along an optical axis.
 11. Anoptical information reproduction apparatus which is equipped with theoptical head apparatus of any one of claims 6 through 9, whichreproduces information from the information medium surface by using theoptical head apparatus.
 12. An optical information recording andreproduction apparatus which is equipped with the optical head apparatusof any one of claims 6 through 9, which records information on and/orreproduces information from the information medium surface by using theoptical head apparatus.